It is known to use a ballast capacitor in lighting applications. If a discharge lamp ballast capacitor does not have a discharge resistor connected across its plates, a charge may be left across the capacitor after the lamp has been turned off. This can be a safety hazard for service personnel. To minimize this safety hazard, Underwriters Laboratories requires that lamp ballast capacitors be equipped with a discharge resistor which will reduce the voltage across the capacitor to 50 volts or less within one minute after power has been turned off.
Where no other suitable impedance is connected with a capacitor within a unit, it is a known practice to provide such a capacitor with a discharge resistor connected across its plates. One unit of this type uses a metal can to which a metal cover is attached. The cover supports two external electrodes for the capacitor, and a discrete discharge resistor is connected between these electrodes. The discharge resistor can be made of carbonloaded paper, or some other conductor/substrate arrangement.
In this and other known devices, the discharge resistor is a discrete unit and must be added to the unit during a separate manufacturing step which requires manual labor. Significant economics could be realized by eliminating such labor-intensive procedures.
Broadly, it is an object of the present invention to produce a discharge lamp ballast capacitor unit having a discharge resistor and to provide a method for manufacturing the same which avoids the shortcomings of the prior art. It is specifically intended that labor intensive manufacturing procedures for incorporating the resistor in the capacitor unit be eliminated.
It is another object of the present invention to provide an integral discharge resistor in a discharge lamp ballast capacitor.
It is yet another object to provide an integral discharge resistor in a discharge lamp ballast capacitor of the wound metallized dielectric type.
It is a further object to provide an effective method for manufacturing a capacitor of the wound metallized dielectric type in such a manner as to provide an integral discharge resistor.
It is also an object of the present invention to provide a capacitor of the type described which is relatively simple and inexpensive in construction, yet reliable and convenient in use and easy to manufacture.
These objects, among others which will be discussed hereinafter, are achieved in a preferred form of the invention, by providing, in a capacitor of the type employing wound metallized dielectric (preferably a metallized film) type, a section of resistive material which extends across the entire width of the film (the axial dimension of the capacitor). A metal contact for one of the capacitor plates (formed by the metallization) is sprayed on either end of the capacitor so as to touch one end of the section of resistive material, thereby creating a resistive shunt across the plates formed on the metallized film.
Advantageously, the section of resistive material is carbon-loaded paper which is secured to the film used to make the capacitor over an unmetallized area. The paper projects beyond either lateral edge of the film so that spraying metal on the ends of the capacitor causes the ends of the paper to be connected to respective capacitor plates.
Because the section of resistive material can be wound into the capacitor as part of the normal winding process, the expense of manufacturing the capacitor with an integral discharge resistor is very low.
The section of resistive material may alternatively be incorporated in a laminated wrapper which is wrapped around the capacitor after winding of the plates has been completed and prior to metal spraying. Also the single section of resistive material may be replaced by a plurality of circumferentially spaced sections all connected in parallel.